In recent years, there is rapid progress of conductive circuits with a high density. The conventional subtractive process used for forming the conductive circuits in which a copper foil laminated on an insulating substrate is etched for patterning thereof requires a prolonged time and is complicated, resulting in production of a large amount of wastes. In consequence, instead of the subtractive process, a printing process or a coating process using a conductive paste comprising conductive particles to form the conductive circuits has been noticed. For example, in a screen printing method generally used for circuit printing, flake-like metal particles having a particle diameter of not less than several micrometers or the like are used as the conductive particles to form a circuit having a thickness of not less than 10 μm and thereby ensure a conductivity thereof. In order to form a circuit having a higher density, still finer metal particles have been developed.
Various kinds of metals may be used as the conductive particles. In view of conductivity and stability with time, among these metals, silver is generally used as the conductive particles. However, silver is not only expensive and a source with a less output, but also has the problem concerning ion migration generated between circuits under high-temperature and high-humidity conditions. Copper has been used as alternative conductive particles in place of silver. However, since copper particles tend to readily form an oxide layer on a surface thereof, there tends to arise such a problem that the copper particles are deteriorated in conductivity owing to the oxide layer. In addition, as the particle size of the copper particles is reduced, the adverse influence of the oxide layer on a conductivity of the particles tends to become more remarkable. In consequence, in order to reduce the oxide layer on the copper particles, it is required that the copper particles are subjected to reducing treatment at a temperature exceeding 300° C. in a reducing atmosphere such as hydrogen or to sintering treatment at a much higher temperature, whereby the conductivity of the copper particles becomes closer to that of a bulk copper. However, even the thus treated copper particles can be used only in limited applications in which an insulating substrate used therewith must be formed of a high heat-resistant material such as ceramic materials and glass.
A conductive paste using a polymer compound as an organic binder is known as a polymer-type conductive paste. The polymer-type conductive paste using the organic binder can ensure fixing of conductive particles and adhesion to a substrate. However, since the organic binder inhibits contact between the conductive particles, the polymer-type conductive paste tends to be deteriorated in conductivity. In general, as the proportion of the conductive particles to the organic binder in the conductive paste is increased, the adhesion of the conductive paste to the substrate is deteriorated, but the conductivity of the conductive paste is enhanced. When the proportion of the conductive particles is further increased, the conductivity of the conductive paste reaches a maximum value and then is decreased owing to increase in voids in the obtained coating film.
The conductive paste using a polymer compound as the organic binder can exhibit a conductivity owing to contact between the conductive particles. The conductivity of even the polymer-type conductive paste using silver particles tends to be reduced to about 1/10 to about 1/1000 time a conductivity of a bulk silver. It is general that the polymer-type conductive paste using copper particles is further deteriorated in conductivity as compared to the silver paste.
In addition, copper tends to rapidly undergo surface oxidation at an elevated temperature. Since the oxidation of copper is accompanied with change in volume of the copper, the copper tends to suffer from occurrence of stress therein, so that adhesion of the copper layer to the substrate generally tends to be deteriorated. The deterioration in adhesion of the copper layer to the substrate tends to become more remarkable as the degree of sintering of the copper particles is increased. Also, the internal stress caused at a boundary between a copper foil such as a copper plating layer and the substrate is larger than that caused at a boundary between a copper powder-containing layer and the substrate. Therefore, in the case where the copper plating layer is formed on the substrate, the deterioration in adhesion of the copper plating layer to the substrate at an elevated temperature tends to become more remarkable than that of the copper powder-containing layer.
In the conventional arts, there has also been proposed the method of enhancing a conductivity of a coating film obtained from a polymer-type conductive paste. For example, in Patent Document 1, it is described that metal fine particles having a particle diameter of not more than 100 nm can be sintered at a temperature far lower than a melting point of a bulk metal to obtain a metal thin film having an excellent conductivity. Also, in Patent Document 2, it is described that a coating film obtained from a metal powder paste is treated with superheated steam. In Patent Document 3, it is described that after treated with superheated steam, a plating layer is formed to obtain a metal thin film.
However, a coating film obtained from a conductive paste comprising metal particles such as copper particles and silver particles is still insufficient in conductivity, and therefore it is required to further improve properties thereof. Further, as the temperature used for treating the coating film with superheated steam is increased, the resulting coating film can exhibit a good conductivity, but there tends to arise such a problem that adhesion of the coating film to an insulating substrate is deteriorated.